#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<stdio.h>
#include<cmath>
#include<iomanip>
using namespace std;
int main()
{
	int n;
	cin >> n;
	double Fn;
	Fn = ((pow((1 + sqrt(5)) / 2, n) - (pow(1 - sqrt(5) / 2, n))) / sqrt(5));
	Fn = round(Fn * 10) / 10;
	cout.setf(ios::fixed);
	cout.precision(2);
	cout << Fn << endl;
	return 0;
}